1. Field of the Invention
The invention relates to a circuit for the field deflection in a picture display device, comprising a generator for generating an essentially sawtooth-shaped variation, a power amplifier coupled to the generator and a field deflection coil connected to an output of the amplifier, the sawtooth generator being provided with a storage element, first means for causing a change of the quantity of information stored in the storage element in one direction during a first part of a field period, and second means for causing a change of the quantity of stored information in the opposite direction during a remaining second part of the field period so that the essentially sawtooth-shaped variation is produced.
2. Description of Related Art
A circuit of this type is generally known and is frequently used in picture display devices. In this circuit, the sawtooth-shaped variation is obtained in that a capacitor is successively charged and discharged. The change of the charge present in the capacitor in a given direction, for example, during discharging of the capacitor is initiated at the instant when a given level is reached during the change in the opposite direction or at the instant when a field signal, for example, a field synchronizing signal present in an incoming video signal occurs. Discharging ends at the instant when the voltage across the capacitor reaches a fixed level, for example, because the capacitor is short-circuited. Embodiments of such circuits are described in U.S. Pat. Nos. 3,422,363 and 4,078,252.
If the first level considered is constant and if the means for reaching this level, for example, a charge current source for the capacitor, is active in the correct manner, or if the successive field synchronizing pulses each time occur at the correct instants, that is to say, if the successive intervals between these pulses each time have the nominal duration of a field period, then the generated sawtooth-shaped voltage has the correct variation. Particularly, if the incoming signals are suitable for display with interlacing, the sawtooth-shaped voltage is too. However, if a variation of the level or of the synchronizing instants occurs, for example, due to an instability at which the edges of the pulses occur irregularly (time jitter), then the level at which the subsequent voltage variation ends, whereafter the capacitor is again charged in this example, does not vary but the corresponding instant does, so that the sawtooth now commencing is not the correct one. This will generally result in an interlacing error.